Central Journal of Pharmacology & Clinical Toxicology Bringing Excellence in Open Access Review Article *Corresponding author Emily Hensleigh, Department of Psychology, University of Nevada Las Vegas, 4505 Maryland Parkway, Las Review: Mechanisms Vegas, NV 89154, USA, Email: Submitted: 15 July 2017 of Methamphetamine Accepted: 25 July 2017 Published: 28 July 2017 ISSN: 2333-7079 Neurotoxicity Copyright Emily Hensleigh* © 2017 Hensleigh Department of Psychology, University of Nevada Las Vegas, USA OPEN ACCESS Keywords Abstract • Methamphetamine Methamphetamine administration results in various behavioral, physical, and neurological • Dopamine effects in both humans and animal species. The outcome and consequences of methamphetamine • Neurotoxicity (METH) administration on neuronal damage depends on the dosage and duration of METH as • Blood brain barrier dysfunction well as additional exogenous and endogenous factors. Prolonged METH administration or high doses of METH result in long term neuronal deficits, mainly in dopamine systems. Several factors contribute to these long term effects of METH on neuronal pathways. This review covers the mechanisms involved in METH neurotoxicity, focusing on hyperthermia, oxidative stress, excitotoxicity, and emerging mechanisms. These effects are discussed in reference to dopamine and, to a lesser extent, serotonin systems chiefly in preclinical models of neurotoxicity. This review begins with a brief summary of the mechanisms of action of METH, the clinical findings in long term METH abusers, and the neuronal markers of METH toxicity. The main sections focus on factors contributing to METH induced neuronal damage including: hyperthermia, oxidative stress, excitotoxicity, and recently identified mechanisms of microglia activation, blood brain barrier dysfunction, and apoptotic pathways. INTRODUCTION dopamine levels will be discussed in more detail in subsequent sections. METH exerts several effects on monoamine systems after acute and long term administration. Acute administration of MARKERS OF METHAMPHETAMINE TOXICITY METH leads to rapid entry into the brain. METH’s chemical In the clinical population, long term METH use leads to resemblance to the monoamines allows METH to enter the presynaptic terminal through the monoamine transport proteins: individuals can develop psychosis characterized by disorganized dopamine transporter (DAT), serotonin transporter (SERT), and thoughtdeficits inand cognitive speech patternsfunctioning. and Afterhallucinations prolonged similar use of to METH, those norepinephrine transporter (NET). METH readily enters into reported in schizophrenia [4]. Individuals who abuse METH also the presynaptic terminal and reverses transport of the vesicular show problems with attention and distractibility [5]. Additionally, monoamine transporter (VMAT2). Reversing the VMAT2 recovering METH abusers exhibit decreased performance in transporter results in reverse transport of neurotransmitters out laboratory tests of executive function and working memory [6,7]. of the synaptic vesicles into the intracellular space [1]. After this Decreased cognitive performance in METH users also correlates occurs, the membrane bound transport proteins reverse their ion with decreased markers of dopamine systems and brain gradient and pump the newly expelled neurotransmitters into the synaptic cleft [2]. This results in elevated levels of dopamine, norepinephrine, and serotonin in the intracellular space and likelymetabolism due to [8,9]. alterations These incombined neurochemical findings systems implicate as along result term of synaptic cleft. prolongedMETH use leadsMETH to use. persistent cognitive deficits. These deficits are Two main mechanisms remove neurotransmitters from the Prolonged administration of METH leads to several synaptic cleft, enzymatic breakdown and reuptake. Monoamine neurochemical alterations observable in humans and animals. oxidase (MAO) and to a lesser extent catechol-O-methyl- Clinical studies indicate alterations in neurotransmitter systems transferase (COMT) break down the monoamines in the synaptic of long term methamphetamine abusers. Wilson [10], looked at cleft and cytosol [3]. Too much dopamine caused by reverse dopamine terminals and protein levels in the brains of deceased transport and inhibition of these enzymes lead to elevated methamphetamine addicts who died from unrelated drug causes. monoamine levels which cause detrimental effects to cell Results indicated decreased tyrosine hydroxylase and dopamine membranes and dopamine terminals. The importance of elevated transporters in the caudate of long term METH abusers, indicating Cite this article: Hensleigh E (2017) Review: Mechanisms of Methamphetamine Neurotoxicity. J Pharmacol Clin Toxicol 5(5):1087. Hensleigh (2017) Email: [email protected] Central Bringing Excellence in Open Access long term alterations within the dopamine system. Utilizing METH. This makes rodents a beneficial model for determining positron emission tomography imaging, several other deficits in HYPERTHERMIAthe mechanisms leading to METH neurotoxicity. dopamine systems have been illustrated. Decreased dopamine transporters and D2 receptors were observed in the prefrontal cortex in individuals with years of prolonged METH use [7,8]. Body temperature mediates the severity of damage to Partial recovery of dopamine transporters was observed in dopamine systems caused by METH in rats and mice. Decreasing METH addicts who were abstinent for twelve months, suggesting an animal’s core body temperature results in less METH induced partial recovery with prolonged abstinence. However, dopamine fatalities. Additionally, rodents which became hyperthermic transporter levels only returned to normal for some, but not all, exhibit more neurotoxic effects compared to those which never subjects. Recovery of dopamine transporter levels additionally became hyperthermic. Several studies suggest body temperature correlate with reported years of METH use [9]. Decreased D2 mediates METH neurotoxicity, but, METH’s neurotoxic effects do receptor binding occurs in the striatum of long term poly drug not require a hyperthermic response. The following discusses users who reported METH as their drug of choice. Increased D3 evidence for the above claims that hyperthermia mediates receptor binding occurs in the midbrain of these same users. neurotoxicity but is not required for neurotoxicity. Increased D3 receptors likely act as a compensatory mechanism Elevated core body temperature results in increased for deficits of decreased dopamine throughout the brain. These dopamine loss in the rat striatum after neurotoxic dosing of receptors also control drug ‘craving’ rather than drug ‘liking’ METH. Bowyer [19], administered neurotoxic doses of METH to suggesting these systems are altered in long term METH users rats and monitored core body temperature finding decreased [11]. However, the role of the D3 receptor in relation to long term dopamine transporters in the striatum three days after the METH use needs further characterization before any conclusions neurotoxic dose of METH with the severity of damage correlating can be made. with body temperatures. Additionally, elevated core body Additional neurotransmitter system deficits are observed temperatures alone did not cause dopamine depletions in the through PET imaging studies. METH users exhibit lower serotonin striatum, suggesting hyperthermia does not cause neurotoxicity levels and decreased glucose metabolism in the prefrontal cortex without METH. Pharmacological interventions (haloperidol, [12,13]. Six week abstinent METH users exhibit decreased VMAT2 diazepam, MK-801) given before a neurotoxic regimen of binding in the striatum [14]. These deficits additionally correlate METH decreased the severity of damage in the striatum. The with cognitive functioning deficits, suggesting long-term METH neuroprotective effects of these pharmacologic interventions use causes neuronal alterations which likely contribute to correlated with the ability of these compounds to lower core cognitive deficits. body temperature. However, during elevated environmental Similar markers of long term METH administration occur temperatures these drugs did not protect against METH-induced in animal models. Wagner [15] observed decreased dopamine dopamine depletions, suggesting, the neuroprotective effects of transporters in the striatum of rats after given two 25.0mg/kg these substances is likely due to their ability to lower core body doses of METH. Rodents given two 15.0mg/kg of METH also temperature [19]. Ali [20], found similar effects in mice. Mice exhibited decreased dopamine metabolite levels in the striatum received a neurotoxic dosage of METH (4 injections of 10mg/ [16]. More recent studies trained rats to self administer METH, an kg every 2 hours) and were kept at normal room temperature arguably better way of modeling human drug use. After given free or a cooler ambient temperature. Those kept at normal room access to METH, rats exhibited decreased expression of tyrosine temperature exhibited decreased markers of dopamine and hydroxylase and dopamine transporters in the striatum and phenobarbitalserotonin in the attenuated striatum whereasdopamine cooler and serotonin ambient temperaturesdepletions in prefrontal cortex [17]. Overall, the effects of METH on serotonin attenuated these deficits. Additionally, pretreatment with systems are less characterized but, after two 15.0mg/kg doses of METH, rats exhibited decreased